In general, a machine having various sliding parts is smoothly operated by supplying a lubricant to these sliding parts. Visual inspection of exposed sliding parts is so easy that the lubricant can be relatively easily timely supplied to the sliding parts of this type in proper quantities in accordance with the result of the inspection. It is difficult, on the other hand, to supply a proper quantity of lubricant to those sliding parts which are not accessible to visual inspection by properly determining the timing for the supply. Conventionally, therefore, a predetermined quantity of lubricant is periodically supplied to those sliding parts inaccessible to visual inspection.
The proper lubricant supply timing varies, however, depending on the operating conditions of the machine. In some cases, therefore, the supply timing may be improper, or the quantity of lubricant supply may be excessive or insufficient, despite the periodical supply of the predetermined quantity of lubricant. If the machine is operated for a long period of time without sufficient lubricant supply, the motion of some sliding parts may lack in smoothness, so that the parts wear irregularly, possibly lowering the accuracy in structure and operation of the machine, and hence, the precision of products. If the lubricant is excessively supplied, on the other hand, there is a possibility of the lubricant leaking and entailing deterioration of those peripheral parts which are poor in resistance to lubricant.